This invention relates to a method of reacting a dianhydride and a diamine in a low-boiling solvent to make a polyimide. In particular, it relates to preparing a polyimide by slowly adding one of the monomers to a solution of the other monomer in a low-boiling, solvent that is heated to a temperature sufficient to fully imidize polyamic acid as soon as it is formed.
In chip scale packaging, semiconductor dies are attached to FR4 or BT substrates using a solution of a polyimidesiloxane adhesive. A temperature below 150xc2x0 C. must be used to protect delicate electronic components. To achieve adhesion below 150xc2x0 C., the solvent in the solution of the polyimidesiloxane adhesive must be removable at a temperature below 150xc2x0 C., which means that solvents such as N-methyl pyrrolidinone (NMP), which boils at 202xc2x0 C., cannot be used.
A polyimidesiloxane is made by reacting a dianhydride with a diamine in a solvent, forming an intermediate polyamic acid. That reaction will occur at room temperature. The solution of the polyamic acid is then heated to about 140 to about 150xc2x0 C. to imidize the polyamic acid. While the intermediate polyamic acid is soluble in polar solvents such as NMP, unfortunately it is not soluble in the low-boiling solvents needed for low temperature adhesive applications, and a gummy precipitate forms. The polyimidesiloxane could be prepared in a high-boiling solvent, such as NMP, precipitated in water, washed, dried, and the solid polyimidesiloxane redissolved in a low-boiling solvent. It would be more convenient, less expensive, and less wasteful, however, to prepare the polyimidesiloxane in the low-boiling solvent and thereby avoid the extra evaporation and redissolving steps.
We have discovered a way to prepare a solution of a polyimide in a low-boiling solvent. In our invention, a solution or slurry is first prepared of one of the monomers in the solvent. That solution is heated to about 80 to about 160xc2x0 C. and the other monomer is slowly added. By preparing the polyimide in this manner, the insoluble polyamic acid intermediate that is formed converts to the soluble polyimide before it can precipitate. Thus, the polyimide can be prepared in the same solvent in which it is to be used and it is not necessary to use one solvent for its preparation and a different solvent for its use.
This invention is applicable to any polyimide that is soluble in a low-boiling solvent as described herein.
The polyimide can be prepared by reacting an aromatic dianhydride with a diamine. Generally, stoichiometric quantities of diamine and dianhydride are used to obtain the highest molecular weight, but the equivalent ratio of dianhydride to diamine can range from 1:2 to 2:1.
Examples of suitable aromatic dianhydrides include:
1,2,5,6-naphthalene tetracarboxylic dianhydride;
1,4,5,8-naphthalene tetracarboxylic dianhydride;
2,3,6,7-naphthalene tetracarboxylic dianhydride;
2-(3xe2x80x2,4xe2x80x2-dicarboxyphenyl)5,6-dicarboxybenzimidazole dianhydride;
2-(3xe2x80x2,4xe2x80x2-dicarboxyphenyl)5,6-dicarboxybenzoxazole dianhydride;
2-(3xe2x80x2,4xe2x80x2-dicarboxyphenyl)5,6-dicarboxybenzothiazole dianhydride;
2,2xe2x80x2,3,3xe2x80x2-benzophenone tetracarboxylic dianhydride;
2,3,3xe2x80x2,4xe2x80x2-benzophenone tetracarboxylic dianhydride;
3,3xe2x80x2,4,4xe2x80x2-benzophenone tetracarboxylic dianhydride (BTDA);
2,2xe2x80x2,3,3xe2x80x2-biphenyl tetracarboxylic dianhydride;
2,3,3xe2x80x2,4xe2x80x2-biphenyl tetracarboxylic dianhydride;
3,3xe2x80x2,4,4xe2x80x2-biphenyl tetracarboxylic dianhydride(BPDA);
bicyclo-[2,2,2]-octen-(7)-2,3,5,6-tetracarboxylic-2,3,5,6-dianhydride;
thio-diphthalic anhydride;
bis(3,4-dicarboxyphenyl)sulfone dianhydride;
bis(3,4-dicarboxyphenyl)sulfoxide dianhydride;
bis(3,4-dicarboxyphenyl oxadiazole-1,3,4)paraphenylene dianhydride;
bis(3,4-dicarboxyphenyl)2,5-oxadiazole 1,3,4-dianhydride;
bis2,5-(3xe2x80x2,4xe2x80x2-dicarboxydiphenylether)1,3,4-oxadiazole dianhydride;
bis(3,4-dicarboxyphenyl)ether dianhydride or 4,4xe2x80x2-oxydiphthalic anhydride (ODPA);
bis(3,4-dicarboxyphenyl)thioether dianhydride;
bisphenol A dianhydride (BPADA);
bisphenol S dianhydride;
2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride or 5,5-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofurandione) (6FDA);
hydroquinone bisether dianhydride;
bis(3,4-dicarboxyphenyl)methane dianhydride;
cyclopentadienyl tetracarboxylic acid dianhydride;
cyclopentane tetracarboxylic dianhydride;
ethylene tetracarboxylic acid dianhydride;
perylene 3,4,9,10-tetracarboxylic dianhydride;
pyromellitic dianhydride (PMDA);
tetrahydrofuran tetracarboxylic dianhydride; and
resorcinol dianhydride.
The dianhydrides can be used in their tetraacid form or as mono, di, tri, or tetra esters of the tetra acid, but the dianhydride form is preferred because it is more reactive. The preferred dianhydride is ODPA because it has been found to give excellent properties. Mixtures of dianhydrides are also contemplated. Additional amounts of monoanhydrides or tri- or higher functional anhydrides can be used to control molecular weight or crosslinking.
The diamine used in preparing the polyimide is preferably aromatic as aromatic diamines give the best properties. Examples of aromatic diamines include:
m- and p-phenylenediamine;
2,4-diaminotoluene (TDA);
2,5- and 2,6-diaminotoluene;
p- and m-xylenediamine;
4,4xe2x80x2-diaminobiphenyl;
4,4xe2x80x2-diaminodiphenyl ether or 4,4xe2x80x2-oxydianiline; (ODA)
3,4xe2x80x2-oxydianiline;
4,4xe2x80x2-diaminobenzophenone;
3,3xe2x80x2,3,4xe2x80x2, or 4,4-diaminophenyl sulfone or m,m-, m,p- or p,p-sulfone dianiline;
4,4xe2x80x2-diaminodiphenyl sulfide;
3,3xe2x80x2-diaminodiphenyl sulfone (APS);
3,3xe2x80x2 or 4,4xe2x80x2-diaminodiphenylmethane or m,m- or p,p-methylene dianiline;
3,3xe2x80x2-dimethylbenzidine;
2,2xe2x80x2-bis[(4-aminophenyl)-1,4-diisopropyl]benzene or 4,4xe2x80x2-isopropylidenedianiline or bisaniline P(BAP);
2,2xe2x80x2-bis[(4-aminophenyl)-1,3-diisopropyl]benzene or 3,3xe2x80x2-isopropylidenedianiline or bisaniline M;
methylene dianiline;
1,4-bis(4-aminophenoxy)benzene;
1,3-bis(4-aminophenoxy)benzene;
1,3-bis(3-aminophenoxy)benzene (APB);
4,4xe2x80x2-bis(4-aminophenoxy)biphenyl;
2,4-diamino-5-chlorotoluene;
2,4-diamino-6-chlorotoluene;
2,2-bis-[4(4-aminophenoxy)phenyl]propane (BAPP);
trifluoromethyl-2,4-diaminobenzene;
trifluoromethyl-3,5-diaminobenzene;
2,2-bis(4-aminophenyl)-hexafluoropropane (6F diamine);
2,2-bis(4-phenoxy aniline)isopropylidene;
2,4,6-trimethyl-1,3-diaminobenzene;
4,4xe2x80x2-diamino-5,5xe2x80x2-trifluoromethyl diphenyloxide;
3,3xe2x80x2-diamino-5,5xe2x80x2-trifluoromethyl diphenyloxide;
4,4xe2x80x2-trifluoromethyl-2,2xe2x80x2-diamino biphenyl;
2,5-dimethyl-1,4-phenylenediamine (DPD);
2,4,6-trimethyl-1,3-diaminobenzene;
diaminoanthraquinone;
4,4xe2x80x2-oxybis[(2-trifluoromethyl)benzeneamine](1,2,4-OBABTF);
4,4xe2x80x2-oxybis[(3-trifluoromethyl)benzeneamine];
4,4xe2x80x2-thiobis[(2-trifluoromethyl)benzeneamine];
4,4xe2x80x2-thiobis[(3-trifluoromethyl)benzeneamine];
4,4xe2x80x2-sulfoxylbis[(2-trifluoromethyl)benzeneamine];
4,4xe2x80x2-sulfoxylbis[(3-trifluoromethyl)benzeneamine];
4,4xe2x80x2-ketobis[(2-trifluoromethyl)benzeneamine];
4,4xe2x80x2-[(2,2,2-trifluoromethyl-1-(trifluoromethyl)-ethylidine)bis(3-trifluoromethyl)benzeneamine]; and
4,4xe2x80x2-dimethylsilylbis[(3-trifluoromethyl)benzeneamine].
The preferred aromatic diamine is APB as it gives excellent properties. Mixtures of aromatic diamines are also contemplated. Additional amounts of monoamines or tri- or higher functional amines can be used to control molecular weight or crosslinking.
The polyimide is preferably a polyimidesiloxane because a polyimidesiloxane has better solubility in the low-boiling solvents used in this invention. To prepare a polyimidesiloxane, a diamine or dianhydride that contains siloxane groups is included as part of the diamine or the dianhydride. A polyimidesiloxane can be made from about 1 to about 80 wt % siloxane-containing monomers and about 20 to about 99 wt % monomers that do not contain siloxane. Preferably, it is made from about 20 to about 60 wt % siloxane-containing monomers and about 40 to about 80 wt % monomers that do not contain siloxane. The siloxane-containing monomer can be either aromatic or non-aromatic, but non-aromatic monomers are preferred as they are more readily available. Examples of siloxane diamines that can be used have the formula: 
Examples of siloxane dianhydrides that can be used have the formula: 
where R1, R2, and R3 are mono, di, and triradicals, respectively, each independently selected from a substituted or unsubstituted 1 to 12 carbon atom aliphatic group or a substituted or unsubstituted 6 to 10 carbon atom aromatic group, where m is an average of 1 to 200. (Siloxane diamines are herein denoted by the notation xe2x80x9cGmxe2x80x9d.) Preferably, m is 1 to 12, R1 is methyl, and R2 is propyl as those compounds are more readily available and work well. Examples of monoradicals include xe2x80x94CH3, xe2x80x94CF3, xe2x80x94CHxe2x95x90CH2, xe2x80x94(CH2)nCF3, xe2x80x94(CF2)nCF3, xe2x80x94C6H5, xe2x80x94CF3xe2x80x94CHFxe2x80x94CF3, and 
Examples of diradicals include xe2x80x94(CH2)nxe2x80x94, xe2x80x94(CH2)nxe2x80x94, xe2x80x94CF2xe2x80x94 and xe2x80x94C6H4xe2x80x94. Example of triradicals include xe2x95x90CHxe2x80x94CH2xe2x80x94, 
Mixtures of siloxane monomers are also contemplated. Siloxane diamines are preferred to siloxane dianhydrides as they are more readily available. To increase solubility in the low-boiling solvent and enhance material properties, the diamine is preferably a mixture of about 5 to about 55 wt % aromatic diamine that does not contain siloxane groups and about 45 to about 95 wt % aliphatic diamine that contains siloxane groups.
To prepare the polyimide, a slurry or solution in a low-boiling organic solvent is formed of either the dianhydride monomer or the diamine monomer, respectively. Because the dianhydride is usually less soluble than the diamine and it is more difficult to add the insoluble dianhydride to a solution of the diamine, it is preferable to form a slurry of the dianhydride monomer and add to it a warmed-up solution of the diamine in some of the low-boiling. The diamine is preferably a mixture of an aromatic diamine that does not contain siloxane groups and an aliphatic diamine that contains siloxane groups. A block copolymer can be formed by adding one of the two diamines to the slurry of the dianhydride before adding the other diamine.
The low-boiling solvent should have a boiling point between about 80 and about 160xc2x0 C. as higher boiling solvents are too difficult to remove and lower boiling solvents evaporate too readily from the adhesive; the preferred boiling point of the solvent is about 120 to about 150xc2x0 C. The invention is applicable to those solvents in which the polyamic acid is insoluble at a temperature below the temperature at which it imidizes, i.e., typically about 140xc2x0 C. Such solvents are usually less polar, i.e., have a dipole moment of less than about 3.5. Examples of such solvents include anisole, toluene, xylene, cyclohexanone, cyclopentanone, methyl ethyl ketone, methyl isobutyl ketone, benzene, hydrocarbons, and mixtures thereof. Anisole, toluene, xylene, methyl ethyl ketone, methyl isobutyl ketone, and mixtures thereof are preferred and anisole is especially preferred because it has a high conversion to the imide, a low drying temperature, a low boiling point, and low toxicity. Toluene, benzene, and xylene form low-boiling azeotropes with the water that is condensed out during imidization. It is therefore preferable to add an azeotroping solvent with another low-boiling solvent to form an azeotrope with the water of imidization that is formed and keep the temperature at which the water is removed by distillation. The amount of azeotroping solvent should be sufficient to form an azeotrope with all of the water that is present; about 5 to about 30 wt % of azeotroping solvent, based on total solvent, is usually adequate.
The solution or slurry of either diamine or dianhydride in the solvent is heated to at least the temperature at which the polyamic acid fully imidizes, typically about 80 to about 160xc2x0 C., and preferably at reflux. At lower temperatures the polyamic acid may precipitate or fail to fully imidize and at higher temperatures the method of this invention is not needed as higher-boiling solvents, such as NMP, can be used. The preferred temperature range is about 120 to about 150xc2x0 C. Higher temperatures with low-boiling solvents can be used if the solution or slurry is under pressure. Sufficient solvent should be used so that the final solution of the polyimide is about 1 to about 40 wt % solids. Less solids require processing too much solvent and higher solids are too viscous. The solution of the polyimide is preferably about 25 to about 35 wt % solids. The other monomer (i.e., dianhydride or diamine) is then added to the solution or slurry, preferably in a small amount of the solvent. This addition is preferably at a rate that is slower than the rate of imidization, typically over about an hour, to avoid clumping and to keep the temperature constant. The dianhydride and the diamine react readily to form a polyamic acid, which almost immediately is converted into a fully (i.e., over 95%) imidized polyimide.
The following examples further illustrate this invention: